The present exemplary embodiments relate to lighting devices incorporating one or more phosphors for the conversion of radiation emitted by a light source. They find particular application in conjunction with converting LED-generated ultraviolet (UV), violet or blue radiation into white light for general illumination purposes. It should be appreciated, however, that the invention is also applicable to the conversion of radiation in other applications, such as Hg-based fluorescent lamps, mercury-free gas discharge lamps, metal halide lamps, UV, violet and/or blue lasers, as well as other white light sources for different applications.
Light emitting diodes (LEDs) are semiconductor light emitters often used as a replacement for other light sources, such as incandescent lamps. They are particularly useful as display lights, warning lights and indicator lights or in other applications where colored light is desired. The color of light produced by an LED is dependent on the type of semiconductor material used in its manufacture.
Colored semiconductor light emitting devices, including light emitting diodes and lasers (both are generally referred to herein as LEDs), have been produced from Group III-V alloys such as gallium nitride (GaN). With reference to the GaN-based LEDs, light is generally emitted in the UV to green range of the electromagnetic spectrum. Until quite recently, LEDs have not been suitable for lighting uses where a bright white light is needed, due to the inherent color of the light produced by the LED.
Recently, techniques have been developed for converting the light emitted from LEDs to useful light for illumination purposes. In one technique, the LED is coated or covered with a phosphor layer. A phosphor is a luminescent material that absorbs radiation energy in a portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. Phosphors of one important class are crystalline inorganic compounds of very high chemical purity and of controlled composition to which small quantities of other elements (called “activators”) have been added to convert them into efficient fluorescent materials. With the right combination of activators and inorganic compounds, the color of the emission can be controlled. Most useful and well-known phosphors emit radiation in the visible portion of the electromagnetic spectrum in response to excitation by electromagnetic radiation outside the visible range.
By interposing a phosphor excited by the radiation generated by the LED, light of a different wavelength, e.g., in the visible range of the spectrum, may be generated. Colored LEDs are often used in toys, indicator lights and other devices. Continuous performance improvements have enabled new applications for LEDs of saturated colors in traffic lights, exit signs, store signs, and the like.
In addition to colored LEDs, a combination of LED generated light and phosphor generated light may be used to produce white light. The most popular white LEDs consist of blue emitting GaInN chips. The blue emitting chips are coated with a phosphor that converts some of the blue radiation to a complementary color, e.g. a yellowish emission. Together, the blue and yellowish radiation produces a white light. There are also white LEDs that utilize a near UV emitting chip and a phosphor blend including red, green and blue emitting phosphors designed to convert the UV radiation to visible light.
Known white light emitting devices comprise a blue light-emitting LED having a peak emission wavelength in the near blue range (from about 440 nm to about 480 nm) combined with a yellow light-emitting phosphor, such as cerium(III) doped yttrium aluminum garnet (“YAG:Ce”), a cerium(III) doped terbium aluminum garnet (“TAG:Ce”), or a europium(II) doped barium orthosilicate (“BOS”). The phosphor absorbs a portion of the radiation emitted from the LED and converts the absorbed radiation to a yellow light. The remainder of the blue light emitted by the LED is transmitted through the phosphor and is mixed with the yellow light emitted by the phosphor. A viewer perceives the mixture of blue and yellow light as a white light. The total of the light from the phosphor material and the LED chip provides a color point with corresponding color coordinates (x and y) and correlated color temperature (CCT), and its spectral distribution provides a color rendering capability, measured by the color rendering index (CRI).
The CRI is commonly defined as a mean value for 8 standard color samples (R1-8), usually referred to as the General Color Rendering Index and abbreviated as Ra, although 14 standard color samples are specified internationally and one can calculate a broader CRI (R1-14) as their mean value. In particular, the R9 value, measuring the color rendering for the strong red, is very important for a range of applications, especially of medical nature. As used herein, “CRI” is used to refer to any of the above general, mean, or special values unless otherwise specified.
Incandescent lamps are known to generate a considerable amount of light in the yellow region of the spectrum, leading to “washing out” of the colors of objects illuminated by them. In response to this, methods have been developed to filter out part of the yellow light through the use of the rare earth element neodymium in the glass envelope. Examples of such lighting systems include the line of REVEAL™ light bulbs produced by GE.
A salient feature of the spectrum of REVEAL™ bulbs is the deep depression in the yellow region, resulting from the filtration by the neodymium glass, as shown in FIG. 5 (solid line). This yellow depression does not extend to zero spectral intensity, but only to about 15% of the highest intensity of the spectrum between 400 and 700 nm. As a result, the REVEAL™ bulbs have a deficiency in the yellow part of the spectrum versus the reference illuminant (shown as a dashed line in FIG. 5). This deficiency leads to increased red-green color contrast when objects are viewed under this lamp, in comparison to a regular incandescent bulb. This has been found to be very appealing to customers.
In the area of LEDs, white phosphor-converted LEDs are based primarily on yellow emitting phosphors, in order to maximize their lumen output. This however reduces the red-green color contrast for test samples of different colors. One way to enhance that contrast (addressed in commonly owned co-pending U.S. patent application Ser. No. 11/285,122) is to use a blend comprising red and green phosphors containing a depression in the yellow region of their combined emission spectrum. Despite the high inherent efficiency, a problem with this solution is its complexity, stemming from the need to use red and green phosphors with sufficient wavelength separation to create the requisite depression in the yellow region, plus an optional variable amount of yellow phosphor to modulate the red-green color contrast enhancement and trade off the latter effect against the loss in luminous efficacy. The invention described herein overcomes this problem by a simpler solution, with the unexpected additional benefit of increased Ra in some cases.
Thus, a continuing need exists to develop new, preferably LED based, lighting devices with enhanced red-green color contrast compared to state of the art solutions.